The use of a chemisorption type filtering media to remove contaminants from air or gas is well known. Typically, the media is held within compartments in filter trays or panels which are mounted in a stack in a housing. For optimum filtering, it is desirable to provide as large a surface area of filtering media to the flow of air or gas as possible. It is known to achieve this by stacking the filter panels in a zigzag or concertina formation, so that one end of each of the panels contacts a respective end of the panel above whilst the other end of the panel contacts an end of the panel below. The flow of air or gas to be filtered is then orientated to flow from one end of the panels to the other (i.e. at a normal to the stack of panels). This provides a large exposed surface area in a relatively small space.
A typical housing comprises a box unit having first and second opposed open ends, and a removable front face. The filter panels can then be stacked in the housing with the ends of the panels locating on grooves or protrusion's in the housing so that each panel is slightly tilted relative to the housing and the panels together form a stack.
It is known to provide filter panels which are formed of a metal or opaque plastics material. The filter media is trapped between upper and lower mesh panels to allow a relatively free flow of air or gas through the filter media.
For maximum operational efficiency of the filter units, it is important that the chemisorption media is renewed before it becomes saturated. The condition of the filter media can be checked by removing it from the filter panels and performing a chemical analysis, but this is costly, messy and time consuming. The media may need to be sent away for specialist checking, which is disadvantageous.
A simple way of checking the condition of the filter media is to use chemisorption type media which changes color, for example, as it is oxidised. The color change produces a simple indication of filter media condition. Nevertheless, when used with prior art metal or opaque plastic filter panels, it is still necessary to dismantle the filter unit and panels in order to check the condition of the filter media.